Show the DBT instrument and add the faucet action

[lmcorbalan]

User story / Problem statement

Currently the UI names Canton Coin throughout: the token constant, the amount formatters, four hardcoded strings, and every doc sentence describing the Amulet path. A fresh wallet account also has no way to get something to vest.

Expected outcome

The dApp presents DBT with its own mark, a dashboard action taps the faucet, and no doc still describes the Amulet path.

Acceptance criteria

• utils/tokens.tsx exports DBT with an inline SVG mark, and canton-coin.png is deleted
• No CC string remains in user-facing copy
• The amount formatters are named for an instrument-neutral figure
• A "Get test tokens" button sits in the dashboard header beside Create
• A tap reports success, or the ledger's own refusal, in a toast
• dapp/frontend/architecture.md, its README and CLAUDE.md, and the root architecture.md describe the registry path

Technical notes

No registry route reports the faucet cap, so the tap amount is a constant that must stay at or under bootstrap's maxPerTap. The root README.md is deliberately untouched: the lines this makes redundant are PR 3's to delete.